DS2777K Maxim Integrated, DS2777K Datasheet - Page 11

• Manufacturer Part Number: DS2777K
• Description: Power Management IC Development Tools
• Manufacturer: Maxim Integrated
• Series: DS2775, DS2776, DS2777, DS2778r

Another DS2776/DS2778 feature is to generate the next
secret from the existing secret, ROM ID, and challenge.
It is important to click Write Challenge before left-click-
ing the Compute Next Secret or Compute Next
Secret with ID button. The device performs the SHA-1
calculation and creates the next secret. The software
performs SHA-1 calculation based on the values in the
text boxes for the secret, ROM ID (if desired), and chal-
lenge and places the new secret, as calculated by soft-
ware, in the Secret text boxes. The new secret is never
read back from the device.
It is important for the software and the DS2776/DS2778
to have identical secrets, ROM IDs, and challenges so
the software can properly verify the operation of the
DS2776/DS2778. If the software is not in sync with the
device, simply clear the secret to get the software and
hardware back in sync.
The DS2776/DS2778 have two commands to compute
the next secret. The Compute Next Secret with ID
command uses the secret, the ROM ID, and the chal-
lenge to perform the SHA-1 encryption algorithm. The
Compute Next Secret command uses the secret and
the challenge, but replaces the ROM ID with 0xFFs to
perform the algorithm. The user can select which com-
mand is used by left-clicking on the appropriate button.
Figure 21. The Challenge
Figure 22. The MAC
Figure 23. Software Computed MAC
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DS2775–DS2778 Evaluation Kits
The ROM ID code is unique for each DS2776/DS2778
device and cannot be changed by the user. The user
can load the device’s ROM ID into the ROM ID text
boxes by left-clicking on the Read ROM – 33h or Read
ROM – 39h buttons, depending on the setting of the
RNAOP bit of the Control register for the DS2776 or by
left-clicking on the READ ROM ID button for the
DS2778. See Figure 20.
If multiple 1-Wire devices are on the bus, the user can
use the Search ROM function from the Net Address
subtab of the Memory tab (Figure 26). Left-click the
Find Devices button and then left-click on the ROM ID
that is desired to be used in the SHA-1 algorithm. The
value of the ROM ID appears in the ROM ID text boxes
on the SHA-1 subtab of the Real Time tab (Figure 12).
The challenge is a random 8-byte block that is used by
the DS2776/DS2778 to perform the SHA-1 encryption
algorithm (Figure 21). Each time the SHA-1 is per-
formed, either during a Compute Next Secret (see
Figure 18) or a Compute MAC (see Figure 22), the
challenge is left in an undefined state. Therefore, the
user must left-click on the Write Challenge button prior
to each computation to get a proper SHA-1 calculation.
The user can left-click on the Randomize Challenge
button to load a random challenge into the Challenge
text boxes. Left-clicking this button does not write the
challenge to the device. It is still required that the user
left-click on the Write Challenge button to write the
challenge to the device.
The MAC is the 20-byte message digest that is the
result of the SHA-1 encryption algorithm (Figure 22).
When the secret has been loaded properly, the ROM ID
has been read, and the challenge has been written to
the device, left-clicking on the Compute MAC or
Compute MAC with ID button performs the SHA-1 cal-
culation, reads back the results, and then displays
them in the MAC text boxes.
The software also performs the SHA-1 calculations
based on the Secret, ROM ID (if desired), and
Challenge text box values and compares its results to
the results read back from the DS2776/DS2778. If the
MAC computed by the software and MAC read back
from the DS2776/DS2778 match, the software displays
“Verified.” If they do not match, “Not Verified” is dis-
played. The user can view the software-computed MAC
by selecting Show Software Computed MAC from the
Tools menu (Figure 8). The user also can compute the
MAC with the DS2776/DS2778, then change one bit in
one of the text boxes of the secret, and then compute
the MAC with software to see how big of a difference
changing one bit makes. See Figure 23.
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